Draw a figure of a man (or woman, whatever you prefer) and draw your hexagram next to it. Connect the bottom line with the feet, the second line with the legs, the third with the belly, the fourth with the arms and back, the fifth with shoulders and face and the top line with the brain. Now you have some clue to the realm of each line. This applies not only to people - situations or ideas also have 'feet', 'legs' and so on.
For several sets of line-places and their meanings see 'line places'.
Most of these ideas date from later ages, but in the YiJing itself there are many lines with parts of the body, and they usually are used as symbols for certain behavior. So the answer can tell you about relations and emotions (2 and 3), or reality, decisions and spirit (1, 4 and 6), and it is not a question of which one is the most important. It is a combination. If you go to the Northpole, you need warm clothes and a lot more. If you go to the beach, you need lemonade and a car, but no warm clothes. Every situation asks for a different set of attributes.
EXAMPLES for the meanings of line-places:
When the top line is yang, one can take a look at hex.23, a hexagram with only the top line yang: leaving or losing outer things, receptive, only the spirit is active. All the other lines or options are receptive. I chose the yang lines for examples, because yang is by its nature more visible than yin.
Line 5 is yang: hex.8: the ability to lead and organize is active, the other abilities are receptive.
Line 4 is yang: hex.16: finding the road is active - motivation, choices, decisions.
Line 3 is yang: hex.15: empathy is active, emotions are strong, but for the rest you follow the situation.
Line 2 is yang: hex.7: conquering the world or defending your territory or interacting with your people.
Bottom line is yang: finding one's own orbit, one's road in life - and walk it in every action.
The trigram-lines and the hexagram-lines were combined with earth, man and heaven, from the bottom up.
For the hexagram lines 1 and 2 belong together (earth), 3 and 4 (man), 5 and 6 (heaven).
Top line heaven-heaven: pure spirit.
Line 5 heaven-man: human side of spirit.
Line 4 man earth: reality of man.
Line 3 man heaven: spirit of man. Spirit is not only what we today call spirit, but all intangible things, in contrast to the matter of earth. So spirit of man is human emotions, thoughts and so on.
Line 2 earth man: human reality.
Bottom line earth earth: pure reality.